Military land vehicles generally have a three, four or five point seat/shoulder belt or harness in one or more of the vehicle seats. Civilian vehicles generally have a three point shoulder belt/lap belt harness. To minimize or reduce injury during a collision or rollover, both military and civilian vehicles have used seat harness pretensioners. In a typical pretensioner, sensors on the vehicle detect the acceleration and/or deceleration forces of a collision or rollover, causing the pretensioners to actuate. The pretensioners then tighten up the seat harness around the seated passenger or driver, milliseconds before the actual full impact or rollover. The tightened harness holds the passenger securely into the seat, reducing impact and rebound injuries. These existing pretensioners are relatively complicated designs because they rely on pyrotechnic devices or electric motors operating a belt retractor, each controlled by an electronic trigger.
Many military land vehicles are designed to resist the destructive forces of mines and improvised explosive devices (IED's). These explosive devices create unique risks to military land vehicles. Detonation of a large explosive device can generate forces on the vehicle exceeding 100 g's, causing the vehicle to accelerate violently upwardly. To reduce the forces on the vehicle crew during a detonation, some military vehicles, such as the Joint Light Tactical Vehicle (JLTV), Bradley Fighting Vehicle, Mine Resistant Ambush Protected Vehicle (MRAP), have blast attenuating seats. Blast attenuating seats are often supported on shock absorbing materials or mounted on a structure, such as a stroking device, that allows the seat to move downward during a blast.
Conventional harness pretensioning techniques are generally not useful with these types of blast attenuating seats, because these seats move relative to the vehicle body during the blast, and because the acceleration is largely vertical, rather than horizontal. In addition, conventional harness pretension techniques require use of pyrotechnic devices or electric retractor motors, and electronic sensors, which can be complicated, costly, and subject to failure due to the harsh operating environment of military vehicles.
Passengers in helicopters are subjected to similar types of extreme forces during crashes or hard landings. Some helicopters also have seats mounted on a stroking device. The engineering challenges presented in harness pretensioning with blast attenuation seats are also present in helicopter seats mounted on stroking devices. Accordingly, it would be advantageous to provide a seat harness pretensioning system for use with vehicle seats that move downwardly to reduce impact on a passenger.